This invention relates to electronic systems for recognizing three-dimensional objects, such as the face of a person; and it relates to the methods by which such systems operate.
One field of use for electronic recognition systems is in the area of security. For example, a person can either be granted access or denied access to a secure area based on whether or not an image of the person's face is electronically recognized by the system. Also, another field of use for electronic recognition systems is in the area of industrial manufacturing. For example, various components which make a product can be selected or rejected by a robotic arm based on whether or not an image of the component is recognized by the system.
In all of the above applications, a key task which needs to be performed as part of the recognition process is that two images must be correlated. One such image is the image of the unknown object which the system is trying to recognize (herein called the access image), and the other image is of the known object which the system uses as a reference (herein called the enrollment image). If the correlation between the access image and the enrollment image exceeds a predetermined threshold value, then the unknown object is recognized.
However, a problem which hinders the correlation step is that a certain degree of misalignment almost always occurs between the access image and the enrollment image. This misalignment occurs because in an operating environment it is impractical to align the known and unknown objects to a camera with exactly the same angles on many different occasions.
As the misalignment between the access image and the enrollment image increases, the correlation between those two images decreases. Consequently, the threshold value to which the correlation is compared, must be lowered in order to recognize an object that is misaligned. However, this presents a dilemma because when the threshold value is lowered, then the probability is increased that the threshold will be exceeded by an object which should not be recognized.
This problem occurs, for example, in the prior art electronic recognition system by Prokoski, et al., which is described in U.S. Pat. No. 5,163,094. In patent '094, an individual who is seeking access through a controlled check point must stop at the entry door and "position himself" relative to a thermal image sensor camera so that a thermal image of his face can be obtained. From that image, a "faceball" is generated; and then an "elemental shape" on a reference faceball is correlated across an area of the currently generated faceball. See patent '094 at column 7, lines 33-37 and lines 46-50.
However, in making this correlation, the area of the currently generated faceball which is correlated to the reference elemental shape must be made large enough to enclose all possible transpositions "which could occur due to twisting, tilting, or tipping of either faceball relative to the other." This is explained in patent '094 at column 7, lines 51-55.
Such twisting, tilting, or tipping of either faceball relative to the other will lessen their degree of correlation. Thus the threshold value for the correlation must be reduced, from what it could be if such twisting, tilting, or tipping did not occur. But, reducing the threshold value increases the probability of the threshold being met by another person who should not be recognized.
Accordingly, a primary object of the present invention is to provide an electronic recognition system in which the above problem is overcome.